Optical coherence tomography (OCT) is an interferometric imaging technique with widespread applications in ophthalmology, cardiology, gastroenterology and other fields of medicine. Huang D, Swanson E A, Lin C P, Schuman J S, Stinson W G, Chang W, Hee M R, Flotte T, Gregory K, Puliafito C A, and Fujimoto J G, “Optical coherence tomography,” Science, Vol 254, 1178-1181 (1991). The ability to view subsurface structures with high resolution (2-15 μm) through small-diameter fiber-optic probes makes OCT especially useful for minimally invasive imaging of internal tissues and organs. Commercially available time-domain OCT systems do not provide sufficient scan speed for unimpeded real-time visualization of organs that move rapidly or that have large surface areas. In the beating heart, for example, OCT imaging of the coronary arteries is a challenge, because imaging must be performed rapidly enough to allow clear visualization of a long segment (>3 cm) of an artery within the interval during which blood is cleared from the field of the view of the probe. The image acquisition rate of the current generation of commercially available OCT systems for coronary artery imaging is limited to approximately 15 images/sec. At this acquisition speed, occlusion of the blood flow with a balloon for at least 30 seconds is required to image a 3-cm segment of the target artery. If the image acquisition rate of OCT systems could be increased by at least an order of magnitude, without significant loss of image quality, balloon occlusion of long periods could be avoided. A segment of an artery could then be imaged by simply injecting a bolus of saline over a few seconds, thereby simplifying the imaging procedure while reducing the risk of myocardial ischemia.
Time-domain OCT systems employ a broadband light source as an input to an interferometer with a mechanically actuated reference arm for path-length scanning. The interference signals generated by reflections from structures at different depths are measured point-by-point as the reference path length changes. In this measurement scheme, the maximum scanning speed is limited both by the dynamic mechanical constraints of the actuator and by the spectral power density of the light source. In such a system using a superluminescent light source that emits an output power of 25 mW over a spectral bandwidth of 40-60 nm, the maximum depth-scanning velocity that can be achieved while maintaining an adequate signal-to-noise ratio for tissue imaging (>90 dB) is approximately 25 m/s. Therefore, 512-line images of a 5 mm deep object can be acquired at a rate no greater than 10 per second.
Frequency-domain (also called Fourier-domain) (FD) OCT overcomes these speed constraints by taking advantage of optical frequency discrimination methods based on Fourier transformation, which eliminate the need for long-range mechanical actuators. Swanson E A and Chinn S R, “Method and Apparatus for Performing Optical Frequency Domain Reflectometry” U.S. Pat. No. 6,160,826 (issued Dec. 12, 2000); Choma M A, Sarunic M V, Yang C, and Izatt J, “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express, Vol. 11, 2183-2189 (2003). Instead of wasting available source power by interrogating the sample point-by-point, FD-OCT collects information from multiple depths simultaneously and discriminates reflections from different depths according to the optical frequencies of the signals they generate. FD-OCT imaging can be accomplished by illuminating the sample with a broadband source and dispersing the reflected light with a spectrometer onto an array detector. Alternatively, the sample can be illuminated with a rapid wavelength-tuned laser and the light reflected during a wavelength sweep collected with a single photodetector. In both cases, a profile of reflections from different depths is obtained by Fourier transformation of the recorded interference signals. Because of their potential to achieve higher performance at lower cost in the 1300 nm spectral region, FD-OCT systems based on swept-frequency laser sources have attracted the most attention for medical applications that require subsurface imaging in highly scattering tissues.
The feasibility of swept-source OCT (SS-OCT) has been demonstrated in several academic research studies. Chinn S R, Swanson E A, and Fujimoto J G, “Optical coherence tomography using a frequency-tunable optical source,” Opt. Lett., Vol. 22, 340-342 (1997); Yun S H, Tearney G J, Bouma B E, Park B H, de Boer J F, “High-speed spectral domain optical coherence tomography at 1.3 μm wavelength,” Optics Express, Vol. 11, pp. 3598-3604 (2003); Choma M A, Hsu K, and Izatt J, “Swept source optical coherence tomography using an all-fiber 1300 nm ring laser source,” J. Biomed. Optics, Vol. 10, p. 044009 (2005); Huber R, Wojtkowski, Taira K, Fujimoto J G, and Hsu K, “Amplified, frequency-swept lasers for frequency domain reflectometry and OCT imaging: design and scaling principles,” Opt. Express, Vol. 13, 3513-3528 (2005). Most of the reported SS-OCT systems employ short-cavity lasers tuned rapidly by an electronically actuated Fabry-Perot filter or a motor-driven grating filter. The implementations disclosed to date suffer from drawbacks that discourage widespread commercialization of SS-OCT. Specifically, current implementations make real-time data acquisition and display difficult, because they employ data acquisition schemes that require post-acquisition re-sampling or interpolation of recorded data before Fourier transformation. In addition, the relatively short coherence length and tendency for mode-hopping of short-cavity lasers reduce signal-to-noise and image resolution at optical scan depths exceeding 2-3 mm. Many medical applications, including coronary artery imaging, require an optical scan depth that exceeds 5 mm.
The recent development of Fourier-Domain Mode Locking (FDML) solves the problem of degraded signal-to-noise and image resolution at large optical scan depths. Huber R, Taira K, and Fujimoto J, “Mode Locking Methods and Apparatus,” US Patent Application No. 2006/0187537, (Published Aug. 24, 2006); Huber R, Wojtkowski M, and Fujimoro J G, “Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography,” Optics Express, Vol. 14, pp. 3225-3237 (2006). However, the practical implementation of a FDML-based SS-OCT system presents several technical challenges. The present invention addresses these challenges and provides solutions to the same.